Why can the 802.11a data rates not be set to all disabled on a SSID?

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I am curious why it is not possible to set as disabled (N/A) all the 802.11a data rates on a SSID.
HiveManager informs us that we must:

"Set at least one data rate as basic (mandatory) for 11a Radio."

In an environment where only 802.11n or 802.11ac clients are to be allowed or expected at 5 GHz, this configuration should, prima facie, be possible.

Am I missing something here?

It is possible to set "Deny 802.11a/b/g clients" to true in a Radio Profile.
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Nick Lowe, Official Rep

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Posted 4 years ago

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Hmmm. Good question. I'll ask around internally to try and find out the rationale. 
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Crowdie, Champ

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If you have 802.11n (or 802.11ac if the radio is 5 GHz) enabled then you must have a mandatory legacy data rate enabled on the same radio.

The reasons for this include:

* The 802.11 standard states that your wireless network must be able to co-exist with other 802.11 network.  If you could disable the legacy data rates and your neighbour had a 802.11bg and/or 802.11a (select as appropriate) your wireless clients would not be able to understand the legacy data rate frames so couldn't read the Duration/ID field to set their NAV.

* Beacons are, generally, transmitted at the lowest mandatory legacy data rate.

This is just a couple of points off the top of my head.  I am sure there are a number more.

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From Aerohive's own blog:


Broadcast frames.

The general rule is that any receiver must be able to decode a broadcast frame. Clause requires that broadcast frames be
transmitted using a basic rate, which, like Beacons, will generally be a
pre-11n rate. To accommodate networks that may have only 802.11n
devices attached, broadcasts can be sent at 11n data rates required of
all 802.11n stations. The 802.11ac amendment allows a network which
requires 802.11ac data rate support to transmit broadcast frames at
802.11ac data rates.

Data communications with other devices. Transmissions
to other devices generally use the same channel bandwidth and a data
rate known to work for the peer. Prior to the adoption of 802.11n, the
rule was to choose a data rate from the peer’s supported data rates.
With the adoption of 802.11n, however, the rules became somewhat more
complicated because of the need to support multiple-stream transmission.

802.11n defined the “non-HT reference rate” of a frame in Table 9-5 of
clause 9.7.9, and requires that control frames generally use the non-HT
reference rate, which is defined by the 802.11a/g equivalent rate for a
given modulation and code rate. For example, a 2-stream 802.11n
transmission in a 40 MHz channel using 16-QAM at R=3/4 will have a data
rate of 180 Mbps. However, the non-HT reference rate is defined by the
modulation and code rate only, and will be 36 Mbps. Control frame
exchanges will be limited to the non-HT reference rate. (802.11ac
extends the non-HT reference table to add a reference rate 54 Mbps to
the two new 256-QAM modulations.)

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Crowdie, Champ

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I went down this path with Cisco around two years ago and that is how I know that 802.11 wireless networks must be able to co-exist.  Just as you have experienced with Aerohive Cisco will not allow you to have a 2.4 and/or 5 GHz radio enabled with no legacy data rates.

I am looking at the 2007 standard and it implies it rather than coming out and saying it:

"This standard...... Permits the operation of an IEEE 802.11-conformant device within a wireless local area network (WLAN) that may coexist with multiple overlapping IEEE 802.11 WLANs."

The 2012 standard has exactly the same text.

The 802.11n standard has a Greenfield mode but I have not found an Enterprise wireless vendor who supports it.  I believe that it has been removed from the 802.11ac standard:

"802.11n offered a "greenfield mode" that saved a few  microseconds in the preamble getting a frame onto the radio link.   Although it was slightly more efficient, it was not a widely adopted feature and was especially avoided in large-scale networks. The  efficiency gains from greenfield mode were often lost because airtime-devouring CTS-to-self messages were required before transmitting in the greenfield mode. As a result, greenfield mode was removed from 802.11ac."

Source: 802.11ac Survival Guide Chapter 3

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Thanks! Looking in to things, I suspect it is the requirement that non-HT rates be used in many cases for control frames that is dictating this. To clarify, I had thought the configuration in HiveOS/HiveManager would just specify the abilty to use these rates outside of the mandatory requirements, but it appears not.

As you point out, Cisco have the same behaviour: